Corrosion-resistant alloys



United States Patent Ofiice 3,246,980 CORROSION-RESISTANT ALLOYS MiltonStern, Indianapolis, Ind., assignor to Union Carbide Corporation, acorporation of N ew York No Drawing. Filed Mar. 23, 1964, Ser. No.354,113 9 Claims. (Cl. 75-134) This is a continuation-in-pa-rt ofapplication S..N. 225,-

851, filed September 24, 1962, and now abandoned, said application S.N.225,851, being a continuation-in-part of application S.N. 821,134, filedJune 18, 1959, now US. Patent No. 3,063,835, granted November 13,1962,said application S.N. 821,134 being a continuation-in-part ofapplication S.N. 732,793, filed May 5, 1958 and now abandoned.

This invention relates to corrosion-resistant alloys and particularly,to corrosion resistant alloys in which chromium is the predominantmetal.

Chromium and many of its alloys, as Well as titanium and many of itsalloys, are noted for excellent resistance to oxidizing corrosive media.In non-oxidizing corrosive media, such as hydrochloric and sulfuric acidsolutions, these metals and alloys exhibit little or no resistance. Itwould be most desirable to have available alloys capabie of withstandingcorrosive environments ranging from strongly oxidizing to Weaklyoxidizing or nonoxidizing. An alloy of this type would have the widestutility. For example, such an alloy would be invaluable in chemicalplant equipment.

Hydrochloric and sulfuric acids are prime examples of non-oxidizingcorrosive media. These acids enter into many important commercialprocesses. They require storing and handling equipment which combine thedesirable properties cited above. If a chromium alloy, for example, wereto possess resistance tov non-oxidizing acid solutions, in addition toits natural endowments of lightness, strength, and resistance tooxidizing acid solutions, it would be extremely useful in suchapplications.

One widely used method for preventing corrosion is to add passivatinginhibitors to the environment which, it is believed, operate byproducing local action current to anodically polarize a metal into thepassive potential region. It is not always desirable, however, to alterthe composition of a solution to prevent corrosion of the equipment inwhich it is contained, particularly when the composition of the solutionis critical or specific to some process. Such a means of preventingcorrosion is limited to the particular case where the composition of thecontained solution is not critical.

According to scientific theory, corrosion resistance can be achieved innon-oxidizing environments by artificially anodically polarizing thebase metal by applying an external current (which might be termed anodicprotection as opposed to cathodic protection). This method, for obviousreasons, would be extremely difficult to accomplish in chemical plantequipment.

It is, therefore, the primary object of this invention to provide alloysof chromium which are resistant to the attack of oxidizing andnon-oxidizing corrosive media.

It is also an object of this invention to provide a method forincreasing the resistance to non-oxidizing corrosive media of the metalchromium, and its alloys.

Other aims and advantages of this invention will be apparent from thefollowing description and the appended claims.

In accordance with these objects a corrosion resistant alloy is providedconsisting essentially of from 0.005 to 5 percent by Weight in theaggregate of at least one metal selected from the group consisting ofruthenium, rhodium, palladium, osmium, iridium, platinum, rhenium, up to10 percent molybdenum, up to 40 percent iron, up to 2 percent copper,and the balance chromium.

A preferred embodiment of the present invention is an alloy consistingessentially of 0.05 to 0.5 percent by weight in the aggregate of atleast one metal selected from the group consisting of ruthenium,rhodium, palladium, osmium, iridium, platinum, rhenium, up to 10 percentmolybdenum, up to 40 percent iron, up to 2 percent copper, and thebalance chromium.

This alloy is characterized by excellent resistance to strong acidsolutions and comparatively lower cost.

It is believed that the important factor in obtaining the desiredimprovement, and the principle upon which the invention operates, liesin the production of sufficient local action current to anodicallypolarize the alloy into the passive region. High local action currentsare created by high potential diiferences between the elecrodes of thegalvanic couple and by shallow polarization curve slopes. The currentrequired to change the potential of either cathode or anode a givenamount by polarization is proportional to the cathode and anode areaswhich, in turn, are determined by the noble metal content of the alloy.

Table I indicates the results of noble metal additions to substantiallypure chromium. Table II provides the same informtaion as to noble metaladditional to binary and tertiary chromium-base alloys. The percentagesgiven in each case are percentages by weight of the total alloy.

TABLE I Eflect of noble metal additions on corrosion rate of chromiumCorrosion Rate (Mils/Year) Nominal Composition Percent Noble MetalBoiling H SO Boiling H01 Cr 1 1 Cr plus 0.1 Pt- 1. 8 1.0 7 E37 Cr plus0.5 Pt. 0 5 2. 7 9 0. 42 Cr plus 1.0 PL 8 0. 46 Cr plus 2.0 Pt. 10 0. 42Cr plus 5.0 Pt. 1. 0.41 Cr plus 0.5 Rh a 2. 0.68 Cr plus 0.5 Os 4 16.0Cr plus 0.5 Pd 0.79 Cr plus 0.5 Ru 0.84 Or plus 0.5 It 0. 46

1 Dissolved.

Patented Apr. 19, 1966 aaaaoeo TABLE 11 chromium-base alloys CorrosionRate (Mils/Year) Nominal Composition Percent Noble Metal Boiling H1804Boiling H01 Cr plus 2 Cu 2060 Or plus 2 Cu plus 0.5 Pt.. 30 84 988 Crplus 10 Mo -e Cr plus 10 Mo plus 0.5 Pt... 8 18 125 0. 7 40 Or plus 40Fe plus 10 Mo 40 Or plus 40 Fe plus 10 Mo plus 0.5 Pt 2 29 130 1Dissolved.

In Table I, the corrosion resistance of chromium Withterms of itsspecific embodiments. Modifications and out any addition is given firstto serve as a comparison. 20 equivalents will be apparent to thoseskilled in the art Pure chromium, it will be noted, dissolves in bothboiling and this disclosure is intended to be illustrative of, butsulfuric acid and boiling hydrochloric acid in the connot necessarily toconstitute a limitation upon, the scope centrations indicated. It willbe observed that all the of the invention. noble metals tested wereextremely effective in reducing What is claimed is: the corrosion rateof chromium. The degree of effec- 1. A corrosion-resistant alloyconsisting essentially of tiveness varies with the quantity of noblemetal added as from about 0.05 to 0.5 percent by weight in the agshownin the tables. It will be observed that as little as gregate of at leastone metal selected from the group con- 0.l% by weight platinum renderschromium practically sisting of ruthenium, rhodium, palladium, osmium,insoluble in 20% boiling sulfuric acid. As shown in iridium, platinum,rhenium, up to 10 percent molybde- Table I, the 0.5 osmium addition,while it decreases 3O num, up to percent iron, up to 2 percent copper,balthe rate drastically, is not as effective as an equal amount ancechromium. of platinum. It will further be observed that, in gen- 2. Acorrosion-resistant alloy consisting essentially of eral, additions ofnoble metal reduce the rate of corabout 0.1 percent by weight platinum,and the balance rosion of chromium in boiling non-oxidizing acids ofconchromium and incidental impurities. centrations as high as 60%. Theproportion of noble 3. A corrosion-resistant alloy consistingessentially of metal addition may be varied according to the anticiabout0.5 percent by weight platinum, and the balance pated use. Where contactwith a more rigorous environchromium and incidental impurities. ment isanticipated, such as strong concentrations of acid, 4. Acorrosion-resistant alloy consisting essentially of more noble metal isrequired. For best results in conabout 0.5 percent by weight palladium,and the balance ditions where exposure to strong solutions isencountered, 40 chromium and incidental impurities. the additive is usedin amounts of between 0.05 and 5. A corrosion-resistant alloy consistingessentially of 0.5%. With amounts below 0.05% noble metal addition,about 10 percent by Weight molybdenum about 0.50 perthe desiredimprovement in corrosion resistance may not cent by weight platinum, andthe balance chromium and be sutlicient to meet the particular needs. Aslittle as incidental impurities.

0.005% of noble metal addition may sufiice for low con- 6. Acorrosion-resistant alloy consisting essentially of centration of acid,however. With amounts substanabout 40 percent by weight iron, about 10percent by tially in excess of about 5%, no marked degree of imweightmolybdenum, about 0.5 percent by weight platiprovement results while thecost of the alloy increases num, and the balance chromium and incidentalimpuriconsiderably. The noble metals. may be present either ties.

singly or in combination with each other. so '7. A corrosion-resistantalloy consisting essentially of In Table II, the corrosion rate ofseveral chromiumabout 2 ercent by weight copper, about 0.50 percent bybase alloys, together With the fate of the Same alloy With weightplatinum, balance chromium and incidental imthe platinum addition isgiven for comparison. It will mbe seen that base aUOYS 9 13 modgrate t8. A corrosion-resistant alloy consisting essentially of strongconcentration of lil'lfil" sulfuric or hydrochlor1c about 0.1 to about05 percent by Weight platinum up acid. However, with the addltron of aslittle as 0.5% to 10 s a A percent by wei ht molybdenum, up to 40percent platinum to the base alloy, a very marked decrease In the b W iht b l" h a d 1 rate of corrosion results. The effectiveness of thenoble e g Iron a C mmmn an mcl enta Imp metal addition, as before,depends on the strength of the L163 g corrosive madium and on the amountof noble metal 9. A corros1on-res1stant alloy consistlng essentially Oradded Moreover some alloys are more effectively about 0.1 to about 0.5percent by weight platinum, about .tected by the same amount of noblemetal addition than by Weight molybdenum about 40 Percent by others. An0.5% addition of platinum, for instance, While Y balance chmmium andincidental p it decreases the rate of corrosion of a chromium-base alloycontaining 2 percent Cu in 40 percent boiling sulfuric References Citedby the Examine! acid, is not as effective as with a chromium-base alloycon- UNITED STATES PATENTS taining 10 percent molybdenum, or an alloyconsisting 1,167,827 1/1916 Kaissr 75 176 of 40 percent iron, 10 percentmolybdenum and the 3 008 854 11/1961 G rant 75l76 balance chromiumAdditions of noble metals may also 3 063 835 11/1962 Stem be made tochromium electrodeposit. 3131059 4/1964 Kaarlel'a 176 The alloys of thepresent invention may be prepared according to current metallurgicalpractice, although the FOREIGN PATENTS invention is not limitedby themethod f preparation. 1 12 7/1914 Great Britain The ingredients may bein any commercially pure form.

The description of the invention above has been in 75 (Other referenceson following page) 3,246,980 5 6 References Cited by the ApplicantHansen: Constitution of Binary Alloys, McGraw-Hill UNITED STATES PATENTSBOOK CO., 1H0, New YOIk, 1959, pages 550, 555.

Pietrokowsky et 211., Journal of Metals, v01. 8, August 2,247,755 7/1941Hensel at 1956, pages 93o 935. Published by the A.I.M.E., New 2,659,66911/1953 Muller. 5 York NY 2,780,545 2/1957 Blank e161.

OTHER REFERENCES BENJAMIN HENKIN, Primary Examiner.

ASM Metals Handbook, 1949 edition, page 22. Pub- WINSTON A, DOUGLAS,Examiner. lished by the ASM, Cleveland, Ohio.

1. A CORROSION-RESISTANT ALLOY CONSISTING ESSENTIALLY OF FROM ABOUT 0.05TO 0.5 PRECENT BY WEIGHT IN THE AGGREGATE OF AT LEAST ONE METAL SELECTEDFROM THE GROUP CONSISTING OF RUTHENIUM, RHODIUM, PALLADIUM, OSMIUM,IRIDIUM, PLATINUM, RHENIUM, UP TO 10 PERCENT MOLYBDENUM, UP TO 40PERCENT IRON, UP TO 2 PERCENT COPPER, BALANCE CHROMIUM.